Electrophoretic displays with controlled amounts of pigment

ABSTRACT

An electrophoretic medium has walls defining a microcavity containing an internal phase. This internal phase comprises electrophoretic particles suspended in a suspending fluid and capable of moving therethrough upon application of an electric field to the electrophoretic medium. The average height of the microcavity differs by not more than about 5 μm from the saturated particle thickness of the electrophoretic particle divided by the volume fraction of the electrophoretic particles in the internal phase.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of copending application Ser. No. 10/837,062, filed Apr. 30, 2004 (Publication No. 2005/0012980), which claims benefit of Provisional Application Ser. No. 60/320,158, filed May 2, 2003, and of Provisional Application Ser. No. 60/320,169, filed May 6, 2003.

This application is also a continuation-in-part of copending application Ser. No. 11/970,811, filed Jan. 8, 2008, which is a continuation of copending application Ser. No. 10/729,044, filed Dec. 5, 2003 (now U.S. Pat. No. 7,352,353, issued Apr. 1, 2008), which is a continuation of application Ser. No. 09/140,860, filed Aug. 27, 1998 (now U.S. Pat. No. 6,710,540, issued Mar. 23, 2004). The aforementioned application Ser. No. 09/140,860 is a continuation-in-part of U.S. Ser. No. 08/504,896 filed Jul. 20, 1995 (now U.S. Pat. No. 6,124,851, issued Sep. 26, 2000), of U.S. Ser. No. 08/983,404 filed Jul. 19, 1997 (now U.S. Pat. No. 7,106,296, issued Sep. 12, 2006), and of U.S. Ser. No. 08/935,800 filed Sep. 23, 1997 (now U.S. Pat. No. 6,120,588, issued Sep. 19, 2000), the contents of all of which are incorporated herein by reference. The aforementioned application Ser. No. 09/140,860 also claims priority to U.S. Ser. No. 60/057,133 filed Aug. 28, 1997, U.S. Ser. No. 60/057,716, filed Aug. 28, 1997, U.S. Ser. No. 60/057,122, filed Aug. 28, 1997, U.S. Ser. No. 60/057,798, filed Aug. 28, 1997, U.S. Ser. No. 60/057,799 filed Aug. 28, 1997, U.S. Ser. No. 60/057,163 filed Aug. 28, 1997, U.S. Ser. No. 60/057,118, filed Aug. 28, 1997, U.S. Ser. No. 60/059,358, filed Sep. 19, 1997, U.S. Ser. No. 60/059,543 filed Sep. 19, 1997, U.S. Ser. No. 60/065,529, filed Nov. 18, 1997, U.S. Ser. No. 60/065,630 filed Nov. 18, 1997, U.S. Ser. No. 60/065,605 filed Nov. 18, 1997, U.S. Ser. No. 60/066,147, filed Nov. 19, 1997, U.S. Ser. No. 60/066,245, filed Nov. 20, 1997, U.S. Ser. No. 60/066,246, filed Nov. 20, 1997, U.S. Ser. No. 60/066,115 filed Nov. 21, 1997, U.S. Ser. No. 60/066,334 filed Nov. 21, 1997, U.S. Ser. No. 60/066,418 filed Nov. 24, 1997, U.S. Ser. No. 60/070,940 filed Jan. 9, 1998, U.S. Ser. No. 60/071,371 filed Jan. 15, 1998, U.S. Ser. No. 60/072,390 filed Jan. 9, 1998, U.S. Ser. No. 60/070,939 filed Jan. 9, 1998, U.S. Ser. No. 60/070,935 filed Jan. 9, 1998, U.S. Ser. No. 60/074,454, filed Feb. 12, 1998, U.S. Ser. No. 60/076,955 filed Mar. 5, 1998, U.S. Ser. No. 60/076,959 filed Mar. 5, 1998, U.S. Ser. No. 60/076,957 filed Mar. 5, 1998, U.S. Ser. No. 60/076,978 filed Mar. 5, 1998, U.S. Ser. No. 60/078,363 filed Mar. 18, 1998, U.S. Ser. No. 60/083,252 filed Apr. 27, 1998, U.S. Ser. No. 60/085,096 filed May 12, 1998, U.S. Ser. No. 60/090,223 filed Jun. 22, 1998, U.S. Ser. No. 60/090,232 filed Jun. 22, 1998, U.S. Ser. No. 60/092,046 filed Jul. 8, 1998, U.S. Ser. No. 60/092,050 filed Jul. 8, 1998, and U.S. Ser. No. 60/093,689 filed Jul. 22, 1998

The entire contents of these patents and copending applications, and of all other U.S. patents and published and copending applications mentioned below, are herein incorporated by reference.

BACKGROUND OF INVENTION

Traditionally, electronic displays such as liquid crystal displays have been made by sandwiching an optoelectrically active material between two pieces of glass. In many cases each piece of glass has an etched, clear electrode structure formed using indium tin oxide. A first electrode structure controls all the segments of the display that may be addressed, that is, changed from one visual state to another. A second electrode, sometimes called a counter electrode, addresses all display segments as one large electrode, and is generally designed not to overlap any of the rear electrode wire connections that are not desired in the final image. Alternatively, the second electrode is also patterned to control specific segments of the displays. In these displays, unaddressed areas of the display have a defined appearance.

Electrophoretic display media, generally characterized by the movement of particles through an applied electric field, are highly reflective, can be made bistable, and consume very little power. Encapsulated electrophoretic displays also enable the display to be printed. These properties allow encapsulated electrophoretic display media to be used in many applications for which traditional electronic displays are not suitable, such as flexible displays. The electro-optical properties of encapsulated displays allow, and in some cases require, novel schemes or configurations to be used to address the displays.

Particle-based electrophoretic displays have been the subject of intense research and development for a number of years. In this type of display, a plurality of charged particles move through a suspending fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. (The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. In practice, some electrophoretic displays, including some of the displays of the present invention, are capable of achieving multiple gray states, and, as demonstrated below, are stable not only in their extreme black and white optical states, but also in their intermediate gray states. Although such displays should properly be described as “multi-stable” rather than “bistable”, the latter term may be used herein for convenience.) The optical property which is changed by application of an electric field is typically color perceptible to the human eye, but may alternatively or in addition, be any one or more of reflectivity, retroreflectivity, luminescence, fluorescence, phosphorescence, or color in the broader sense of meaning a difference in absorption or reflectance at non-visible wavelengths. Problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.

Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation have recently been published describing encapsulated electrophoretic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a liquid suspending medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. Encapsulated media of this type are described, for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564; 6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989; 6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790; 6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182; 6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949; 6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; 6,580,545; 6,639,578; 6,652,075; 6,657,772; 6,664,944; 6,680,725; 6,683,333; 6,704,133; 6,710,540; 6,721,083; 6,724,519; 6,727,881; 6,738,050; 6,750,473; 6,753,999; 6,816,147; 6,819,471; 6,822,782; 6,825,068; 6,825,829; 6,825,970; 6,831,769; 6,839,158; 6,842,167; 6,842,279; 6,842,657; 6,864,875; 6,865,010; 6,866,760; 6,870,661; 6,900,851; 6,922,276; 6,950,200; 6,958,848; 6,967,640; 6,982,178; 6,987,603; 6,995,550; 7,002,728; 7,012,600; 7,012,735; 7,023,420; 7,030,412; 7,030,854; 7,034,783; 7,038,655; 7,061,663; 7,071,913; 7,075,502; 7,075,703; 7,079,305; 7,106,296; 7,109,968; 7,110,163; 7,110,164; 7,116,318; 7,116,466; 7,119,759; 7,119,772; 7,148,128; 7,167,155; 7,170,670; 7,173,752; 7,176,880; 7,180,649; 7,190,008; 7,193,625; 7,202,847; 7,202,991; 7,206,119; 7,223,672; 7,230,750; 7,230,751; 7,236,790; 7,236,792; 7,242,513; 7,247,379; 7,256,766; 7,259,744; 7,280,094; 7,304,634; 7,304,787; 7,312,784; 7,312,794; 7,312,916; 7,237,511; 7,339,715; 7,349,148; 7,352,353; 7,365,394; and 7,365,733; and U.S. Patent Applications Publication Nos. 2002/0060321; 2002/0090980; 2003/0102858; 2003/0151702; 2003/0222315; 2004/0105036; 2004/0112750; 2004/0119681; 2004/0155857; 2004/0180476; 2004/0190114; 2004/0257635; 2004/0263947; 2005/0000813; 2005/0007336; 2005/0012980; 2005/0018273; 2005/0024353; 2005/0062714; 2005/0099672; 2005/0122284; 2005/0122306; 2005/0122563; 2005/0134554; 2005/0151709; 2005/0152018; 2005/0156340; 2005/0179642; 2005/0190137; 2005/0212747; 2005/0253777; 2005/0280626; 2006/0007527; 2006/0038772; 2006/0139308; 2006/0139310; 2006/0139311; 2006/0176267; 2006/0181492; 2006/0181504; 2006/0194619; 2006/0197737; 2006/0197738; 2006/0202949; 2006/0223282; 2006/0232531; 2006/0245038; 2006/0262060; 2006/0279527; 2006/0291034; 2007/0035532; 2007/0035808; 2007/0052757; 2007/0057908; 2007/0069247; 2007/0085818; 2007/0091417; 2007/0091418; 2007/0109219; 2007/0128352; 2007/0146310; 2007/0152956; 2007/0153361; 2007/0200795; 2007/0200874; 2007/0201124; 2007/0207560; 2007/0211002; 2007/0211331; 2007/0223079; 2007/0247697; 2007/0285385; 2007/0286975; 2007/0286975; 2008/0013155; 2008/0013156; 2008/0023332; 2008/0024429; 2008/0024482; 2008/0030832; 2008/0043318; 2008/0048969; 2008/0048970; 2008/0054879; 2008/0057252; and 2008/0074730; and International Applications Publication Nos. WO 00/38000; WO 00/36560; WO 00/67110; and WO 01/07961; and European Patents Nos. 1,099,207 B1; and 1,145,072 B1.

Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called polymer-dispersed electrophoretic display, in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.

An encapsulated electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; electrophoretic deposition (See U.S. Pat. No. 7,339,715); and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.

A related type of electrophoretic display is a so-called “microcell electrophoretic display”. In a microcell electrophoretic display, the charged particles and the fluid are not encapsulated within microcapsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, U.S. Pat. Nos. 6,672,921 and 6,788,449, both assigned to Sipix Imaging, Inc.

Hereinafter, the term “microcavity electrophoretic display” will be used to cover both encapsulated and microcell electrophoretic displays.

Known microcavity electrophoretic displays can be divided into two main types, referred to hereinafter for convenience as “single particle” and “dual particle” respectively. A single particle medium has only a single type of electrophoretic particle suspended in a colored medium, at least one optical characteristic of which differs from that of the particles. (In referring to a single type of particle, we do not imply that all particles of the type are absolutely identical. For example, provided that all particles of the type possess substantially the same optical characteristic and a charge of the same polarity, considerable variation in parameters such as particle size and electrophoretic mobility can be tolerated without affecting the utility of the medium.) When such a medium is placed between a pair of electrodes, at least one of which is transparent, depending upon the relative potentials of the two electrodes, the medium can display the optical characteristic of the particles (when the particles are adjacent the electrode closer to the observer, hereinafter called the “front” electrode) or the optical characteristic of the suspending medium (when the particles are adjacent the electrode remote from the observer, hereinafter called the “rear” electrode, so that the particles are hidden by the colored suspending medium).

A dual particle medium has two different types of particles differing in at least one optical characteristic and a suspending fluid which may be uncolored or colored, but which is typically uncolored. The two types of particles differ in electrophoretic mobility; this difference in mobility may be in polarity (this type may hereinafter be referred to as an “opposite charge dual particle” medium) and/or magnitude. When such a dual particle medium is placed between the aforementioned pair of electrodes, depending upon the relative potentials of the two electrodes, the medium can display the optical characteristic of either set of particles, although the exact manner in which this is achieved differs depending upon whether the difference in mobility is in polarity or only in magnitude. For ease of illustration, consider an electrophoretic medium in which one type of particles are black and the other type white. If the two types of particles differ in polarity (if, for example, the black particles are positively charged and the white particles negatively charged), the particles will be attracted to the two different electrodes, so that if, for example, the front electrode is negative relative to the rear electrode, the black particles will be attracted to the front electrode and the white particles to the rear electrode, so that the medium will appear black to the observer. Conversely, if the front electrode is positive relative to the rear electrode, the white particles will be attracted to the front electrode and the black particles to the rear electrode, so that the medium will appear white to the observer.

If the two types of particles have charges of the same polarity, but differ in electrophoretic mobility (this type of medium may hereinafter to referred to as a “same polarity dual particle” medium), both types of particles will be attracted to the same electrode, but one type will reach the electrode before the other, so that the type facing the observer differs depending upon the electrode to which the particles are attracted. For example suppose the previous illustration is modified so that both the black and white particles are positively charged, but the black particles have the higher electrophoretic mobility. If now the front electrode is negative relative to the rear electrode, both the black and white particles will be attracted to the front electrode, but the black particles, because of their higher mobility, will reach it first, so that a layer of black particles will coat the front electrode and the medium will appear black to the observer. Conversely, if the front electrode is positive relative to the rear electrode, both the black and white particles will be attracted to the rear electrode, but the black particles, because of their higher mobility will reach it first, so that a layer of black particles will coat the rear electrode, leaving a layer of white particles remote from the rear electrode and facing the observer, so that the medium will appear white to the observer: note that this type of dual particle medium requires that the suspending fluid to sufficiently transparent to allow the layer of white particles remote from the rear electrode to be readily visible to the observer. Typically, the suspending fluid in such a display is not colored at all, but some color may be incorporated for the purpose of correcting any undesirable tint in the white particles seen therethrough.

Certain of the aforementioned E Ink and MIT patents and applications describe electrophoretic media which have more than two types of electrophoretic particles within a single capsule. For present purposes, such multi-particle media are regarded as a sub-class of dual particle media.

Both single and dual particle electrophoretic displays may be capable of intermediate gray states having optical characteristics intermediate the two extreme optical states already described.

Microcavity electrophoretic displays may have microcavities of any suitable shape; for example, several of the aforementioned E Ink and MIT patents and applications (see especially U.S. Pat. Nos. 6,067,185 and 6,392,785) describe encapsulated electrophoretic displays in which originally-spherical capsules are flattened so that they have substantially the form of oblate ellipsoids. When a large number of such oblate ellipsoidal capsules are deposited upon a substrate, the walls of the capsules may contact one another, until the capsules approach a close-packed condition in which the walls of adjacent capsules are flattened against one another so that the capsules assume substantially the form of polygonal prisms. In theory, in a close-packed layer of capsules, the individual capsules would have the form of hexagonal prisms, and indeed micrographs of some encapsulated electrophoretic media show a close approach to this condition. However, more typically the individual capsules have substantially the form of irregular polygonal prisms. In polymer-dispersed encapsulated electrophoretic media, there are of course no individual capsules, but the droplets of internal phase may assume forms similar to the capsule forms already discussed.

Thus, microcavities in microcavity electrophoretic displays may be irregular. The following discussion will consider microcavities in a laminar film having substantial dimensions in a plane considered as having X and Y axes, and a much smaller dimension perpendicular to this plane, this dimension being denoted the Z axis. The average internal height of the microcavity along the Z axis will be denoted the “internal phase height” or “IP height” of the microcavity. The average area parallel to the XY plane of the microcavity (averaged along the Z axis) excluding capsule or cavity walls will be denoted the “IP area”, while the corresponding average area including the capsule or cavity walls will be denoted the “capsule area”. The maximum diameter parallel to the XY plane of the microcavity at any height excluding capsule or cavity walls will be denoted the “IP diameter”, while the corresponding average diameter including the capsule or cavity walls will be denoted the “capsule diameter”.

It has long been known that, to optimize the optical performance of electrophoretic and other electro-optic displays, it is desirable to maximize the active fraction of the display area, i.e., the fraction of the display area which can change optical state when an electric field is applied to the electro-optic medium. Inactive areas of the display, such as the black masks often used in liquid crystal displays, and the area occupied by capsule or microcavity walls in microcavity electrophoretic displays, do not change optical state when an electric field is applied, and hence reduce the contrast between the extreme optical states of the display. However, there is relatively little consideration in the published literature relating to other parameters affecting the optical performance of electrophoretic displays, and in particular the amount of pigment needed in the electrophoretic medium. This may be due, in part, to the fact that most electrophoretic displays discussed in the literature have been single particle electrophoretic displays, and in such displays the limiting factor on the thickness of the electrophoretic medium is normally the optical density of the dye in the suspending fluid, and not the amount of pigment present. This is not the case with dual particle electrophoretic displays, and may not be the case with single particle displays using dyes with optical densities higher than those used in most prior art electrophoretic displays.

It has now been found that the optical performance of electrophoretic displays is substantially affected by variations in the amount of pigment present in the electrophoretic medium, the IP height of the medium, and the pigment loading of the internal phase (i.e., the proportion of the volume of the internal phase which is comprised of pigment), and this invention relates to electrophoretic media and displays in which the relationships among these various parameters are controlled so as to improve, and desirably to optimize, the optical performance of the media and displays.

SUMMARY OF INVENTION

Accordingly, this invention provides an electrophoretic medium having walls defining at least one microcavity containing an internal phase, this internal phase comprising a plurality of at least one type of electrophoretic particle suspended in a suspending fluid and capable of moving therethrough upon application of an electric field to the electrophoretic medium, the average height of the at least one microcavity differing by not more than about 5 μm from the saturated particle thickness of the electrophoretic particle divided by the volume fraction of the electrophoretic particles in the internal phase.

The term “saturated particle thickness” of electrophoretic particles in a microcavity is used herein to denote the thickness of the layer of particles which would be formed over the IP area of the microcavity using an internal phase containing just sufficient electrophoretic particles that, if application of a specific electric field to the medium for a time T suffices to switch the electrophoretic medium between its extreme optical states, variations in the time of application of this specific electric field within the range of 0.95 to 1.05 T will not change the optical properties of either extreme state of the electrophoretic medium by more than 2 units of L*, where L* has the usual CIE definition. This saturated particle thickness is calculated without regard to packing factors; in other words, the saturated particle thickness is the hypothetical thickness of the layer which would be formed over the IP area if the electrophoretic particles formed a completely solid layer, without voids, over this area. For example, if an electrophoretic medium has an IP height of 50 μm and contains 10 percent by volume of electrophoretic particles, its saturated particle thickness is 5 μm. As will readily be apparent to those familiar with the packing of multi-particle layers, this thickness does not correspond to the actual thickness of the layer of particles formed when all the particles are driven to one end surface of the microcavity, since inevitably this particle layer will contain a substantial volume fraction of voids. For the sake of simplicity, suppose the electrophoretic medium comprises spherical particles of essentially uniform diameter which form an essentially close-packed layer. Since the packing fraction for close-packed uniform spheres is approximately 0.64, the actual thickness of the layer formed on one end surface of the microcavity will be about 5/0.64 or 7.8 μm.

In preferred forms of the present invention, variations in the time of application of this specific electric field within the range of 0.9 to 1.1 T will not change the optical properties of either extreme state of the electrophoretic medium by more than 2 units of L*, and in especially preferred forms of the invention variations in the time of application of this specific electric field within the range of 0.8 to 1.2 T will not change the optical properties of either extreme state of the electrophoretic medium by more than 2 units of L*.

The saturated particle thickness is typically between about 1 and about 5 μm, and desirably between about 1.5 and about 2.5 μm. The volume fraction of electrophoretic particles in the internal phase (i.e., the fraction of the volume of the internal phase occupied by the electrophoretic particles) is typically from 3 to 40 percent, and desirably in the range to 6 to 18 percent.

The electrophoretic medium of the present invention may be of any of the types described above. Thus, the electrophoretic medium may be a single particle medium comprising a single type of electrophoretic particle in a colored suspending fluid. Alternatively, the electrophoretic medium may be a dual particle medium comprising a first type of electrophoretic particle having a first optical characteristic and a first electrophoretic mobility and a second type of electrophoretic particle having a second optical characteristic different from the first optical characteristic and a second electrophoretic mobility different from the first electrophoretic mobility. In such a dual particle medium, the suspending fluid may be uncolored. The electrophoretic medium may be of the microcell type, in which the electrophoretic particles and the suspending fluid are retained within a plurality of cavities formed within a carrier medium. Alternatively, the electrophoretic medium may be an encapsulated electrophoretic medium, in which the electrophoretic particles and the suspending fluid are held within a plurality of capsules.

One type of display in which the present invention may be especially useful is the so-called “shutter mode” microcavity display. A shutter mode microcavity display is a microcavity display having one “opaque” optical state in which the display (or any given pixel thereof) displays the color or other optical characteristic of an electrophoretic particle, and a second optical state in the which the electrophoretic medium or pixel thereof is light-transmissive. Such a shutter mode display may be of the single or dual particle type, and may have more than the two specified optical states; for example, a dual particle shutter mode display using black and white electrophoretic particles, may have a black opaque state, a white opaque state and a light-transmissive state. The light-transmissive state of a shutter mode display is typically produced by confining the electrophoretic particles in a minor proportion of the cross-sectional area of each microcavity so that light is free to pass through the major proportion of this cross-sectional area. The confinement of the electrophoretic particles to the minor proportion of the cross-sectional area may be effected by using a shaped microcavity (see, for example, the aforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798), by placement of electrodes in specific positions relative to the microcavity (see, for example, the aforementioned U.S. Pat. No. 7,170,670 and Japanese Published Patent Applications Nos. 2002-174828 and 2001-356374), or by dielectrophoretic driving of the electrophoretic particles (see, for example, U.S. Pat. No. 7,259,744).

Although the second optical state of a shutter mode display has been referred to above as “light-transmissive”, a shutter mode display may incorporate a colored or uncolored reflector adjacent the microcavity medium and on the opposed side thereof from that normally by an observer (this opposed surface hereinafter for convenience being referred to as the “rear surface” of the microcavity medium) so that (as described for example in the aforementioned U.S. Pat. No. 7,259,744) the light-transmissive optical state of the display actually displays the color (if any) of the reflector. In particular, an advantageous form of color microcavity display may be formed by providing a backplane having a plurality of pixel electrodes, forming a color filter or reflector on the backplane, and then forming a layer of a shutter mode microcavity medium over the color filter or reflector. A microcell medium might be formed by photolithographic techniques, by forming a layer of photoresist over the color filter, and exposing and developing in the conventional manner to form cells walls separating a plurality of microcells. Alternatively, a layer, typically a polymer layer, might be provided over the color filter and microcavities formed mechanically therein, or a preformed layer containing microcavities provided over the color filter. In either case, the microcavities formed can be filled with an electrophoretic mixture (electrophoretic particles plus suspending fluid) and sealed.

Regardless of the exact method used for its manufacture and the exact type of electrophoretic medium employed, this type of color shutter mode display has the advantages that positioning the color filter with respect to the pixel electrodes is simplified, since the pixel electrodes are readily visible during formation or attachment of the filter, and, more importantly, that the positioning of the color filter adjacent the pixel electrodes avoids visible artifacts which may occur due to parallax when a color filter substantially separated from a backplane (for example, a color filter on the opposed side of the electrophoretic medium from the backplane) is viewed off-axis.

One problem with such shutter mode microcavity displays is ensuring good contrast ratio, since even in the light-transmissive optical state of such a shutter mode display, the minor proportion of each microcavity occupied by the electrophoretic particles still displays the color of those particles (or a mixture of the relevant colors, in the case of a dual particle display), and this continuing display of the color of the electrophoretic particles reduces the contrast ratio. The present invention enables one to control the amount of electrophoretic particles needed in a microcavity display, thus minimizing the proportion of each microcavity occupied by the electrophoretic particles in the light-transmissive state of the display and maximizing the contrast ratio, while still providing sufficient electrophoretic particles to ensure good optical properties in the first optical state of the display.

A preferred white electrophoretic particle for use in the present electrophoretic media comprises titania (TiO₂). If the electrophoretic medium is of the dual particle type, it may further comprise dark colored particles formed from carbon black or copper chromite, the dark colored particles formed from carbon black or copper chromite and having an electrophoretic mobility different from the electrophoretic mobility of the titania particles.

Useful embodiments of the present invention may have an IP height between about 10 and about 30 μm and a volume fraction of electrophoretic particles of between about 3 and about 15 percent. Preferred embodiments have an IP height between about 12 and about 25 μm and a volume fraction of electrophoretic particles of between about 5 and about 12 percent. The viscosity of the internal phase is typically less than about 5 mPa sec, and typically greater than about 1 mPa sec.

This invention extends to an electrophoretic display comprising an electrophoretic medium of the present invention and at least one electrode disposed adjacent the electrophoretic medium and arranged to apply an electric field thereto. Typically, such an electrophoretic display will have a rear electrode structure having a plurality of electrodes arranged to apply an electric field to the electrophoretic medium.

In another aspect, this invention provides an electrophoretic suspension intended for use in an electrophoretic display and comprising more than about 5 percent by weight of white particles, the suspension having a viscosity of from about 2 to about 7 mPa sec.

This electrophoretic suspension of the present invention may include any of the preferred features of the electrophoretic medium of the present invention, as already described.

The present invention can provide a highly-flexible, reflective display which can be manufactured easily, consumes little (or no in the case of bistable displays) power, and can, therefore, be incorporated into a variety of applications. The invention features a printable display comprising an encapsulated electrophoretic display medium. The resulting display is flexible. Since the display media can be printed, the display itself can be made inexpensively.

An encapsulated electrophoretic display can be constructed so that the optical state of the display is stable for some length of time. The definition of a bistable state depends on the application for the display. A slowly-decaying optical state can be effectively bistable if the optical state is substantially unchanged over the required viewing time. For example, in a display which is updated every few minutes, a display image which is stable for hours or days is effectively bistable for that application. In this application, the term bistable also indicates a display with an optical state sufficiently long-lived as to be effectively bistable for the application in mind. Alternatively, it is possible to construct encapsulated electrophoretic displays in which the image decays quickly once the addressing voltage to the display is removed (i.e., the display is not bistable or multistable). As will be described, in some applications it is advantageous to use an encapsulated electrophoretic display which is not bistable. Whether or not an encapsulated electrophoretic display is bistable, and its degree of bistability, can be controlled through appropriate chemical modification of the electrophoretic particles, the suspending fluid, the capsule, and binder materials.

An encapsulated electrophoretic display may take many forms. The display may comprise capsules dispersed in a binder. The capsules may be of any size or shape. The capsules may, for example, be spherical and may have diameters in the millimeter range or the micron range, but is preferably from ten to a few hundred microns. The capsules may be formed by an encapsulation technique, as described below. Particles may be encapsulated in the capsules. The particles may be two or more different types of particles. The particles may be colored, luminescent, light-absorbing or transparent, for example. The particles may include neat pigments, dyed (laked) pigments or pigment/polymer composites, for example. The display may further comprise a suspending fluid in which the particles are dispersed.

The successful construction of an encapsulated electrophoretic display requires the proper interaction of several different types of materials and processes, such as a polymeric binder and, optionally, a capsule membrane. These materials must be chemically compatible with the electrophoretic particles and fluid, as well as with each other. The capsule materials may engage in useful surface interactions with the electrophoretic particles, or may act as a chemical or physical boundary between the fluid and the binder.

As already indicated, in some cases, the encapsulation step of the process is not necessary, and the electrophoretic fluid may be directly dispersed or emulsified into the binder (or a precursor to the binder materials) and an effective “polymer-dispersed electrophoretic display” constructed. In such displays, voids created in the binder may be referred to as capsules or microcapsules even though no capsule membrane is present. The binder dispersed electrophoretic display may be of the emulsion or phase separation type.

Throughout the specification, reference will be made to printing or printed. As used throughout the specification, printing is intended to include all forms of printing and coating, including: premetered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, and curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; and other similar techniques. A “printed element” refers to an element formed using any one of the above techniques.

This invention provides novel methods and apparatus for controlling and addressing particle-based displays. Additionally, the invention discloses applications of these methods and materials on flexible substrates, which are useful in large-area, low cost, or high-durability applications.

In one aspect, the present invention relates to an encapsulated electrophoretic display. The display includes a substrate and at least one capsule containing a highly-resistive fluid and a plurality of particles. The display also includes at least two electrodes disposed adjacent the capsule, a potential difference between the electrodes causing some of the particles to migrate toward at least one of the two electrodes. This causes the capsule to change optical properties.

In another aspect, the present invention relates to a colored electrophoretic display. The electrophoretic display includes a substrate and at least one capsule containing a highly-resistive fluid and a plurality of particles. The display also includes colored electrodes. Potential differences are applied to the electrodes in order to control the particles and present a colored display to a viewer.

In yet another aspect, the present invention relates to an electrostatically addressable display comprising a substrate, an encapsulated electrophoretic display adjacent the substrate, and an optional dielectric sheet adjacent the electrophoretic display. Application of an electrostatic charge to the dielectric sheet or display material modulates the appearance of the electrophoretic display.

In still another aspect, the present invention relates to an electrostatically addressable encapsulated display comprising a film and a pair of electrodes. The film includes at least one capsule containing an electrophoretic suspension. The pair of electrodes is attached to either side of the film. Application of an electrostatic charge to the film modulates the optical properties.

In still another aspect, the present invention relates to an electrophoretic display that comprises a conductive substrate, and at least one capsule printed on such substrate. Application of an electrostatic charge to the capsule modulates the optical properties of the display.

In still another aspect the present invention relates to a method for matrix addressing an encapsulated display. The method includes the step of providing three or more electrodes for each display cell and applying a sequence of potentials to the electrodes to control movement of particles within each cell.

In yet another aspect, the present invention relates to a matrix addressed electrophoretic display. The display includes a capsule containing charged particles and three or more electrodes disposed adjacent the capsule. A sequence of voltage potentials is applied to the three or more electrodes causing the charged particles to migrate within the capsule responsive to the sequence of voltage potentials.

In still another aspect, the present invention relates to a rear electrode structure for electrically addressable displays. The structure includes a substrate, a first electrode disposed on a first side of the substrate, and a conductor disposed on a second side of the substrate. The substrate defines at least one conductive via in electrical communication with both the first electrode and the conductor.

BRIEF DESCRIPTION OF DRAWINGS

The invention is pointed out with particularity in the appended claims. The advantages of the invention described above, together with further advantages, may be better understood by referring to the following description taken in conjunction with the accompanying drawings. In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention.

FIG. 1A is a diagrammatic side view of an embodiment of a rear-addressing electrode structure for a particle-based display in which the smaller electrode has been placed at a voltage relative to the large electrode causing the particles to migrate to the smaller electrode.

FIG. 1B is a diagrammatic side view of an embodiment of a rear-addressing electrode structure for a particle-based display in which the larger electrode has been placed at a voltage relative to the smaller electrode causing the particles to migrate to the larger electrode.

FIG. 1C is a diagrammatic top-down view of one embodiment of a rear-addressing electrode structure.

FIG. 2A is a diagrammatic side view of an embodiment of a rear-addressing electrode structure having a retroreflective layer associated with the larger electrode in which the smaller electrode has been placed at a voltage relative to the large electrode causing the particles to migrate to the smaller electrode.

FIG. 2B is a diagrammatic side view of an embodiment of a rear-addressing electrode structure having a retroreflective layer associated with the larger electrode in which the larger electrode has been placed at a voltage relative to the smaller electrode causing the particles to migrate to the larger electrode.

FIG. 2C is a diagrammatic side view of an embodiment of a rear-addressing electrode structure having a retroreflective layer disposed below the larger electrode in which the smaller electrode has been placed at a voltage relative to the large electrode causing the particles to migrate to the smaller electrode.

FIG. 2D is a diagrammatic side view of an embodiment of a rear-addressing electrode structure having a retroreflective layer disposed below the larger electrode in which the larger electrode has been placed at a voltage relative to the smaller electrode causing the particles to migrate to the larger electrode.

FIG. 3A is a diagrammatic side view of an embodiment of an addressing structure in which a direct-current electric field has been applied to the capsule causing the particles to migrate to the smaller electrode.

FIG. 3B is a diagrammatic side view of an embodiment of an addressing structure in which an alternating-current electric field has been applied to the capsule causing the particles to disperse into the capsule.

FIG. 3C is a diagrammatic side view of an embodiment of an addressing structure having transparent electrodes, in which a direct-current electric field has been applied to the capsule causing the particles to migrate to the smaller electrode.

FIG. 3D is a diagrammatic side view of an embodiment of an addressing structure having transparent electrodes, in which an alternating-current electric field has been applied to the capsule causing the particles to disperse into the capsule.

FIG. 4A is a diagrammatic side view of an embodiment of a rear-addressing electrode structure for a particle-based display in which multiple smaller electrodes have been placed at a voltage relative to multiple larger electrodes, causing the particles to migrate to the smaller electrodes.

FIG. 4B is a diagrammatic side view of an embodiment of a rear-addressing electrode structure for a particle-based display in which multiple larger electrodes have been placed at a voltage relative to multiple smaller electrodes, causing the particles to migrate to the larger electrodes.

FIG. 5A is a diagrammatic side view of an embodiment of a rear-addressing electrode structure for a particle-based display having colored electrodes and a white electrode, in which the colored electrodes have been placed at a voltage relative to the white electrode causing the particles to migrate to the colored electrodes.

FIG. 5B is a diagrammatic side view of an embodiment of a rear-addressing electrode structure for a particle-based display having colored electrodes and a white electrode, in which the white electrode has been placed at a voltage relative to the colored electrodes causing the particles to migrate to the white electrode.

FIG. 6 is a diagrammatic side view of an embodiment of a color display element having red, green, and blue particles of different electrophoretic mobilities.

FIGS. 7A-7B depict the steps taken to address the display of FIG. 6 to display red.

FIGS. 8A-8D depict the steps taken to address the display of FIG. 6 to display blue.

FIGS. 9A-9C depict the steps taken to address the display of FIG. 6 to display green.

FIG. 10 is a perspective embodiment of a rear electrode structure for addressing a seven segment display.

FIG. 11 is a perspective embodiment of a rear electrode structure for addressing a three by three matrix display element.

FIG. 12 is a cross-sectional view of a printed circuit board used as a rear electrode addressing structure.

FIG. 13 is a cross-sectional view of a dielectric sheet used as a rear electrode addressing structure.

FIG. 14 is a cross-sectional view of a rear electrode addressing structure that is formed by printing.

FIG. 15 is a perspective view of an embodiment of a control grid addressing structure.

FIG. 16 is an embodiment of an electrophoretic display that can be addressed using a stylus.

DETAILED DESCRIPTION

As already indicated, it has been found that, in microcavity electrophoretic displays, there is an optimum IP height related to two key variables, namely the saturated particle thickness of the electrophoretic particles, i.e., the minimum thickness of each pigment to achieve an adequate optical state, and the volume fraction of that pigment in the internal phase of the display.

At first glance, it might appear that achieving an “adequate optical state” in an electrophoretic display is solely a function of the desired optical property of the pigments used for any given application. However, it has been found that if an electrophoretic medium does not contain sufficient pigment, the optical properties of the medium may be adversely affected; for example, if an electrophoretic medium contains insufficient white pigment, the reflectivity of the white state of the medium may be lower than the same state of a similar medium containing more white pigment.

For various technical reasons, it is generally desirable to keep an electrophoretic medium as thin as possible consistent with good optical properties. Since the rate at which electrophoretic particles move is determined by electric field strength, and since (all other factors being equal) the electric field strength in an electrophoretic display is proportional to the voltage applied between the electrodes divided by the distance between these electrodes, it is generally desirable to keep this distance to a minimum (i.e., to keep the electrophoretic medium as thin as possible) in order to keep the operating voltage as low as possible, a low operating voltage being desirable to reduce energy consumption by the display (especially in portable, battery-driven devices) and to minimize the cost and complexity of electronic circuitry needed to drive the display. Also, keeping the electrophoretic medium as thin as possible reduces the distance which the electrophoretic particles need to travel during switching of the display between its extreme optical states and thus, at a constant electric field, increases the switching speed of the display. Also, in certain applications, electrophoretic displays are attractive because they can be made flexible, and it is easier to produce a flexible display with a thin electrophoretic medium. Hence, it might at first glance appear that an electrophoretic display should have a minimum IP height and a high volume fraction of pigment in the internal phase so as to provide sufficient pigment to ensure an optimum optical state when that pigment is visible.

However, there are some countervailing considerations. Increased pigment loading will typically result in higher viscosity of the internal phase, and this higher viscosity reduces electrophoretic particle velocity and slows the switching speed of the display for a given applied electric field.

Thus, the optimum formulation of an electrophoretic medium for any particular combination of pigment(s), suspending fluid, operating voltage and desired switching time is a complicated matter. The situation is further complicated by the complex relationships between applied voltages and optical states in electrophoretic media. As discussed in the aforementioned U.S. Pat. No. 7,012,600 and several other of the aforementioned E Ink and MIT patents and applications, electrophoretic media do not act as simple voltage transducers (as do liquid crystals) but rather act, to a first approximation, as impulse transducers, so that the final state of a pixel depends not only upon the electric field applied but also upon the state of the pixel prior to the application of the electric field. This type of behavior can cause serious complications when it is desired to produce an area of supposedly uniform color on a display. Consider, for example, a black and white display intended for use in reading black text, with or without illustrations, on a white background. When such a display is re-written (i.e., when a new page is displayed), unless both the electrophoretic medium formulation and the drive scheme employed are carefully chosen, there may be small variations among the optical states of the numerous pixels in the supposedly uniform white background, and the human eye is very sensitive to such small variations in optical states in a supposedly uniform area, especially since readers are accustomed to a highly uniform white background on a printed page.

In accordance with the present invention it has been found that, to secure good optical performance from a microcavity electrophoretic medium, it is important to correlate the average height of the microcavities with the saturated particle thickness (as defined above) and the volume fraction of the electrophoretic particles in the internal phase of the electrophoretic medium. It has been found that there is an optimum IP height for microcavity electrophoretic media. If the saturation thickness of a pigment is T and the volume fraction of the pigment in the internal phase is F, the optimum IP height is T/F, and in practice it is desirable that the actual IP height not differ from this optimum value by more than about 5 μm. When an electrophoretic medium contains two or more pigments, the value of T/F should be calculated separately for each pigment, and the optimum IP height set to the largest of the resultant values.

It will readily be apparent that, in an electrophoretic medium containing more than one type of electrophoretic particle, each type of particle will have its own saturated particle thickness. Single particle electrophoretic media typically comprise a white pigment in a dyed suspending fluid, while dual particle electrophoretic media typically comprise white and black particles in an uncolored suspending fluid. However, in both cases, the critical saturated particle thickness is usually that of the white particles, since the white particles scatter light while the black particles absorb it, and the pigment thickness needed to scatter light is greater than that required to absorb light.

It has been found that, if the microcavities in an electrophoretic medium have an IP height significantly greater than this optimum T/F value, display performance is reduced. One reason is that the distance that the pigment must travel in order to reach the microcavity wall is greater. A second reason is that for a given voltage field across the internal phase, the field strength is reduced. A low field strength reduces particle velocity. Furthermore, in a multi-pigment system in which particles of opposite charge may have a tendency to aggregate, a low field strength reduces the number of aggregates that are separated. On the other hand, if the microcavities have an IP height significantly less than this optimum value, the desired optical state may not be achieved due to insufficient optical density of pigment.

To evaluate various electrophoretic media, one can measure the total pigment “saturation thickness”, achieved from a pulse length time and electric field level across an internal phase, under which a change in the pulse period would change the optical properties of the pigment by no more than a desired threshold amount for visual artifacts. In a typical system with a typical white/black switching speed around 300 ms, at a saturated thickness a pulse length change of 50 ms at 15V would change the optical properties by less than 2 L*. In systems employing faster switching electrophoretic media, a saturated thickness may be adequate if the optical properties would change by less than 2 L* for pulse length variation about 5-20% of the typical white/black switch speed of the medium. As already mentioned, for purposes of clear definition, the term “saturated particle thickness” of electrophoretic particles in a microcavity is used herein to denote the thickness (assuming 100 percent packing) of the layer of particles which would be formed over the IP area of the microcavity using an internal phase containing just sufficient electrophoretic particles that, if application of a specific electric field to the medium for a time T suffices to switch the electrophoretic medium between its extreme optical states, variations in the time of application of this specific electric field within the range of 0.95 to 1.05 T (i.e., variations in time of ±5 percent) will not change the optical properties of either extreme state of the electrophoretic medium by more than 2 units of L*. Desirably, the system should withstand variations in time of ±10 percent, and preferably ±20 percent, without changes in optical properties exceeding 2 L*.

A preferred white pigment for use in electrophoretic media is titania. The titania desirably has a surface coating of silica and/or alumina, and is also desirably polymer coated, as described in the aforementioned U.S. Pat. No. 6,822,782 or in the related U.S. Pat. No. 7,230,750. As is known in the art of pigments and paints, the titania particles should desirable be between 0.1 μm and 0.5 μm in diameter, and ideally between 0.2 μm and 0.4 μm in diameter, for greatest efficiency in scattering with minimal thickness. However a composite particle may also be used that contains multiple pigment particles. Titania particles, especially those described in the aforementioned U.S. Pat. Nos. 6,822,782 and 7,230,750, can have saturated particle thicknesses in the range of from about 1 to 10 μm, and desirably from about 1 to about 5 μm, depending somewhat upon the addressing waveform used. The present inventors have found that, in one preferred titania/carbon black dual particle electrophoretic medium of the present invention, using a preferred addressing pulse of 15 V and a pulse length of between 200 and 500 ms, the titania provided adequate coverage levels at thicknesses between 1.5 and 2.5 μm. In another preferred electrophoretic medium having a lower viscosity, faster switching internal phase driven by 15 V pulses with a pulse length of 100 ms, the titania also provided adequate coverage at thicknesses between 1.5 and 2.5 μm.

Copper chromite particles may be used in place of carbon black particles as the dark colored particles in dual particles media of the present invention (or, indeed, in single particle media where a dark particle is desired). The preparation and use of copper chromite particles in electrophoretic media is fully described in U.S. Pat. No. 7,002,728.

As discussed above, at first glance it appears desirable to formulate thin electrophoretic media with high pigment loadings, but the ability to do so is limited by the increase in viscosity associated with high pigment loadings. Some increase in pigment loading may be advantageous, as compared with pigment loadings used in prior art electrophoretic displays. For example, to achieve whiter systems, it can be moderately useful to increase titania loading to higher levels such as 5-7 μm thickness. Also, whereas some electrophoretic suspensions known in the art have employed a pigment loading of less than 2 percent by weight, it has been found that an internal phase comprising up to 45 percent by weight or 15 percent by volume of titania particles can have a viscosity that permits the particles to achieve an adequate velocity under electric fields of a strength useful in commercial devices, so as to enable the devices to use driving voltages typically 15 V or less. Preferred media of the present invention may typically have a titania loading of 5 to 15 percent by volume with an internal phase viscosity between 1 and 6 mPa sec. Given a saturated particle thickness of 1.5 to 2.5 μm, and a titania loading of 10 percent by volume, it has been found that visual artifacts are reduced when the IP height for a microcavity is between 15 μm and 25 μm, with the optimum value being substantially 20 μm.

Other types of internal phases may permit reduced viscosity, thus permitting a higher pigment loading. For example, an internal phase using a gaseous suspending fluid (see, for example, 2004/0112750) would be able to support a much higher pigment loading and correspondingly a lower IP height. Such gas-based phases could function with particle loadings as high as 90 percent by volume.

It is believed (although the invention is in no way limited by this belief) that one of the reasons for the improved optical states achieved by the present invention is that if, in a microcavity electrophoretic display, the pigment is not sufficiently thick, the display is vulnerable to image ghosting. The reasons for such image ghosting may include small voltage variations in the addressing system, slowly-decaying remnant voltages or polarization in the microcavities of the display, settling of the pigments over time, improper mixing of various pigments, and differences between the RC time constant of the internal phase and its external environment, including any binder present. All of these effects can cause variations in the amount of pigment visible to an observer, superimposed on the variations intended to be caused by the addressing of the display.

The “visual artifact level” of a display (typically a high resolution display) may be measured by any suitable means. In one method, many pixels are each subjected to a different switching history typical of the intended usage model. The greatest optical difference between any two pixels is the “maximum visual artifact level.” Alternatively, a single pixel may be subjected to many different switching histories and a consistent test addressing pulse then applied. The greatest optical difference between the resulting optical states is another way to measure the “maximum visual artifact level.”

To achieve consistent image quality with minimal visual artifacts, it is desirable that the electrophoretic medium contain a minimum adequate thickness of the pigment such that a small variation in pigment level has a minimal optical effect. For portable high-resolution display applications, this optical effect should ideally be no more than 1 to 2 L* units, given typical variations in actual pigment packing thicknesses.

Having described in detail the features peculiar to the “controlled pigment” media and displays of the present invention, more general guidance regarding the construction and uses of electrophoretic media will now be given.

An electronic ink is an optoelectronically active material which comprises at least two phases: an electrophoretic contrast media phase and a coating/binding phase. The electrophoretic phase comprises, in some embodiments, a single species of electrophoretic particles dispersed in a clear or dyed medium, or more than one species of electrophoretic particles having distinct physical and electrical characteristics dispersed in a clear or dyed medium. In some embodiments the electrophoretic phase is encapsulated, that is, there is a capsule wall phase between the two phases. The coating/binding phase includes, in one embodiment, a polymer matrix that surrounds the electrophoretic phase. In this embodiment, the polymer in the polymeric binder is capable of being dried, crosslinked, or otherwise cured as in traditional inks, and therefore a printing process can be used to deposit the electronic ink onto a substrate. An electronic ink is capable of being printed by several different processes, depending on the mechanical properties of the specific ink employed. For example, the fragility or viscosity of a particular ink may result in a different process selection. A very viscous ink would not be well-suited to deposition by an inkjet printing process, while a fragile ink might not be used in a knife over roll coating process.

The optical quality of an electronic ink is quite distinct from other electronic display materials. The most notable difference is that the electronic ink provides a high degree of both reflectance and contrast because it is pigment based (as are ordinary printing inks). The light scattered from the electronic ink comes from a very thin layer of pigment close to the top of the viewing surface. In this respect it resembles an ordinary, printed image. Also, electronic ink is easily viewed from a wide range of viewing angles in the same manner as a printed page, and such ink approximates a Lambertian contrast curve more closely than any other electronic display material. Since electronic ink can be printed, it can be included on the same surface with any other printed material, including traditional inks. Electronic ink can be made optically stable in all display configurations, that is, the ink can be set to a persistent optical state. Fabrication of a display by printing an electronic ink is particularly useful in low power applications because of this stability.

Electronic ink displays are novel in that they can be addressed by DC voltages and draw very little current. As such, the conductive leads and electrodes used to deliver the voltage to electronic ink displays can be of relatively high resistivity. The ability to use resistive conductors substantially widens the number and type of materials that can be used as conductors in electronic ink displays. In particular, the use of costly vacuum-sputtered indium tin oxide (ITO) conductors, a standard material in liquid crystal devices, is not required. Aside from cost savings, the replacement of ITO with other materials can provide benefits in appearance, processing capabilities (printed conductors), flexibility, and durability. Additionally, the printed electrodes are in contact only with a solid binder, not with a fluid layer (like liquid crystals). This means that some conductive materials, which would otherwise dissolve or be degraded by contact with liquid crystals, can be used in an electronic ink application. These include opaque metallic inks for the rear electrode (e.g., silver and graphite inks), as well as conductive transparent inks for either substrate. These conductive coatings include semiconducting colloids, examples of which are indium tin oxide and antimony-doped tin oxide. Organic conductors (polymeric conductors and molecular organic conductors) also may be used. Polymers include, but are not limited to, polyaniline and derivatives, polythiophene and derivatives, poly(3,4-ethylenedioxythiophene) (PEDOT) and derivatives, polypyrrole and derivatives, and polyphenylenevinylene (PPV) and derivatives. Organic molecular conductors include, but are not limited to, derivatives of naphthalene, phthalocyanine, and pentacene. Polymer layers can be made thinner and more transparent than with traditional displays because conductivity requirements are not as stringent.

As an example, there are a class of materials called electroconductive powders which are also useful as coatable transparent conductors in electronic ink displays. One example is Zelec ECP electroconductive powders from Du Pont Chemical Co. of Wilmington, Del.

Referring now to FIGS. 1A and 1B, an addressing scheme for controlling particle-based displays is shown in which electrodes are disposed on only one side of a display, allowing the display to be rear-addressed. Utilizing only one side of the display for electrodes simplifies fabrication of displays. For example, if the electrodes are disposed on only the rear side of a display, both of the electrodes can be fabricated using opaque materials, because the electrodes do not need to be transparent.

FIG. 1A depicts a single capsule 20 of an encapsulated display media. In brief overview, the embodiment depicted in FIG. 1A includes a capsule 20 containing at least one particle 50 dispersed in a suspending fluid 25. The capsule 20 is addressed by a first electrode 30 and a second electrode 40. The first electrode 30 is smaller than the second electrode 40. The first electrode 30 and the second electrode 40 may be set to voltage potentials which affect the position of the particles 50 in the capsule 20.

The particles 50 represent 0.1% to 20% of the volume enclosed by the capsule 20. In some embodiments the particles 50 represent 2.5% to 17.5% of the volume enclosed by capsule 20. In preferred embodiments, the particles 50 represent 5% to 15% of the volume enclosed by the capsule 20. In more preferred embodiments the particles 50 represent 9% to 11% of the volume defined by the capsule 20. In general, the volume percentage of the capsule 20 that the particles 50 represent should be selected so that the particles 50 expose most of the second, larger electrode 40 when positioned over the first, smaller electrode 30. As described in detail below, the particles 50 may be colored any one of a number of colors. The particles 50 may be either positively charged or negatively charged.

The particles 50 are dispersed in a dispersing fluid 25. The dispersing fluid 25 should have a low dielectric constant. The fluid 25 may be clear, or substantially clear, so that the fluid 25 does not inhibit viewing the particles 50 and the electrodes 30, 40 from position 10. In other embodiments, the fluid 25 is dyed. In some embodiments the dispersing fluid 25 has a specific gravity matched to the density of the particles 50. These embodiments can provide a bistable display media, because the particles 50 do not tend to move in certain compositions absent an electric field applied via the electrodes 30, 40.

The electrodes 30, 40 should be sized and positioned appropriately so that together they address the entire capsule 20. There may be exactly one pair of electrodes 30, 40 per capsule 20, multiple pairs of electrodes 30, 40 per capsule 20, or a single pair of electrodes 30, 40 may span multiple capsules 20. In the embodiment shown in FIGS. 1A and 1B, the capsule 20 has a flattened, rectangular shape. In these embodiments, the electrodes 30, 40 should address most, or all, of the flattened surface area adjacent the electrodes 30, 40. The smaller electrode 30 is at most one-half the size of the larger electrode 40. In preferred embodiments the smaller electrode is one-quarter the size of the larger electrode 40; in more preferred embodiments the smaller electrode 30 is one-eighth the size of the larger electrode 40. In even more preferred embodiments, the smaller electrode 30 is one-sixteenth the size of the larger electrode 40. It should be noted that reference to “smaller” in connection with the electrode 30 means that the electrode 30 addresses a smaller amount of the surface area of the capsule 20, not necessarily that the electrode 30 is physically smaller than the larger electrode 40. For example, multiple capsules 20 may be positioned such that less of each capsule 20 is addressed by the “smaller” electrode 30, even though both electrodes 30, 40 are equal in size. It should also be noted that, as shown in FIG. 1C, electrode 30 may address only a small corner of a rectangular capsule 20 (shown in phantom view in FIG. 1C), requiring the larger electrode 40 to surround the smaller electrode 30 on two sides in order to properly address the capsule 20. Selection of the percentage volume of the particles 50 and the electrodes 30, 40 in this manner allow the encapsulated display media to be addressed as described below.

Electrodes may be fabricated from any material capable of conducting electricity so that electrode 30, 40 may apply an electric field to the capsule 20. As noted above, the rear-addressed embodiments depicted in FIGS. 1A and 1B allow the electrodes 30, 40 to be fabricated from opaque materials such as solder paste, copper, copper-clad polyimide, graphite inks, silver inks and other metal-containing conductive inks. Alternatively, electrodes may be fabricated using transparent materials such as indium tin oxide and conductive polymers such as polyaniline or polythiophenes. Electrodes 30, 40 may be provided with contrasting optical properties. In some embodiments, one of the electrodes has an optical property complementary to optical properties of the particles 50.

In one embodiment, the capsule 20 contains positively charged black particles 50, and a substantially clear suspending fluid 25. The first, smaller electrode 30 is colored black, and is smaller than the second electrode 40, which is colored white or is highly reflective. When the smaller, black electrode 30 is placed at a negative voltage potential relative to larger, white electrode 40, the positively-charged particles 50 migrate to the smaller, black electrode 30. The effect to a viewer of the capsule 20 located at position 10 is a mixture of the larger, white electrode 40 and the smaller, black electrode 30, creating an effect which is largely white. Referring to FIG. 1B, when the smaller, black electrode 30 is placed at a positive voltage potential relative to the larger, white electrode 40, particles 50 migrate to the larger, white electrode 40 and the viewer is presented a mixture of the black particles 50 covering the larger, white electrode 40 and the smaller, black electrode 30, creating an effect which is largely black. In this manner the capsule 20 may be addressed to display either a white visual state or a black visual state.

Other two-color schemes are easily provided by varying the color of the smaller electrode 30 and the particles 50 or by varying the color of the larger electrode 40. For example, varying the color of the larger electrode 40 allows fabrication of a rear-addressed, two-color display having black as one of the colors. Alternatively, varying the color of the smaller electrode 30 and the particles 50 allow a rear-addressed two-color system to be fabricated having white as one of the colors. Further, it is contemplated that the particles 50 and the smaller electrode 30 can be different colors. In these embodiments, a two-color display may be fabricated having a second color that is different from the color of the smaller electrode 30 and the particles 50. For example, a rear-addressed, orange-white display may be fabricated by providing blue particles 50, a red, smaller electrode 30, and a white (or highly reflective) larger electrode 40. In general, the optical properties of the electrodes 30, 40 and the particles 50 can be independently selected to provide desired display characteristics. In some embodiments the optical properties of the dispersing fluid 25 may also be varied, e.g. the fluid 25 may be dyed.

In other embodiments the larger electrode 40 may be reflective instead of white. In these embodiments, when the particles 50 are moved to the smaller electrode 30, light reflects off the reflective surface 60 associated with the larger electrode 40 and the capsule 20 appears light in color, e.g. white (see FIG. 2A). When the particles 50 are moved to the larger electrode 40, the reflecting surface 60 is obscured and the capsule 20 appears dark (see FIG. 2B) because light is absorbed by the particles 50 before reaching the reflecting surface 60. The reflecting surface 60 for the larger electrode 40 may possess retroflective properties, specular reflection properties, diffuse reflective properties or gain reflection properties. In certain embodiments, the reflective surface 60 reflects light with a Lambertian distribution. The surface 60 may be provided as a plurality of glass spheres disposed on the electrode 40, a diffractive reflecting layer such as a holographically formed reflector, a surface patterned to totally internally reflect incident light, a brightness-enhancing film, a diffuse reflecting layer, an embossed plastic or metal film, or any other known reflecting surface. The reflecting surface 60 may be provided as a separate layer laminated onto the larger electrode 40 or the reflecting surface 60 may be provided as a unitary part of the larger electrode 40. In the embodiments depicted by FIGS. 2C and 2D, the reflecting surface may be disposed below the electrodes 30, 40 vis-à-vis the viewpoint 10. In these embodiments, electrode 30 should be transparent so that light may be reflected by surface 60. In other embodiments, proper switching of the particles may be accomplished with a combination of alternating-current (AC) and direct-current (DC) electric fields and described below in connection with FIGS. 3A-3D.

In still other embodiments, the rear-addressed display previously discussed can be configured to transition between largely transmissive and largely opaque modes of operation (referred to hereafter as “shutter mode”). Referring back to FIGS. 1A and 1B, in these embodiments the capsule 20 contains at least one positively-charged particle 50 dispersed in a substantially clear dispersing fluid 25. The larger electrode 40 is transparent and the smaller electrode 30 is opaque. When the smaller, opaque electrode 30 is placed at a negative voltage potential relative to the larger, transmissive electrode 40, the particles 50 migrate to the smaller, opaque electrode 30. The effect to a viewer of the capsule 20 located at position 10 is a mixture of the larger, transparent electrode 40 and the smaller, opaque electrode 30, creating an effect which is largely transparent. Referring to FIG. 1B, when the smaller, opaque electrode 30 is placed at a positive voltage potential relative to the larger, transparent electrode 40, particles 50 migrate to the second electrode 40 and the viewer is presented a mixture of the opaque particles 50 covering the larger, transparent electrode 40 and the smaller, opaque electrode 30, creating an effect which is largely opaque. In this manner, a display formed using the capsules depicted in FIGS. 1A and 1B may be switched between transmissive and opaque modes. Such a display can be used to construct a window that can be rendered opaque. Although FIGS. 1A-2D depict a pair of electrodes associated with each capsule 20, it should be understood that each pair of electrodes may be associated with more than one capsule 20.

A similar technique may be used in connection with the embodiment of FIGS. 3A, 3B, 3C, and 3D. Referring to FIG. 3A, a capsule 20 contains at least one dark or black particle 50 dispersed in a substantially clear dispersing fluid 25. A smaller, opaque electrode 30 and a larger, transparent electrode 40 apply both direct-current (DC) electric fields and alternating-current (AC) fields to the capsule 20. A DC field can be applied to the capsule 20 to cause the particles 50 to migrate towards the smaller electrode 30. For example, if the particles 50 are positively charged, the smaller electrode is placed a voltage that is more negative than the larger electrode 40. Although FIGS. 3A-3D depict only one capsule per electrode pair, multiple capsules may be addressed using the same electrode pair.

The smaller electrode 30 is at most one-half the size of the larger electrode 40. In preferred embodiments the smaller electrode is one-quarter the size of the larger electrode 40; in more preferred embodiments the smaller electrode 30 is one-eighth the size of the larger electrode 40. In even more preferred embodiments, the smaller electrode 30 is one-sixteenth the size of the larger electrode 40.

Causing the particles 50 to migrate to the smaller electrode 30, as depicted in FIG. 3A, allows incident light to pass through the larger, transparent electrode 40 and be reflected by a reflecting surface 60. In shutter mode, the reflecting surface 60 is replaced by a translucent layer, a transparent layer, or a layer is not provided at all, and incident light is allowed to pass through the capsule 20, i.e. the capsule 20 is transmissive.

Referring now to FIG. 3B, the particles 50 are dispersed into the capsule 20 by applying an AC field to the capsule 20 via the electrodes 30, 40. The particles 50, dispersed into the capsule 20 by the AC field, block incident light from passing through the capsule 20, causing it to appear dark at the viewpoint 10. The embodiment depicted in FIGS. 3A-3B may be used in shutter mode by not providing the reflecting surface 60 and instead providing a translucent layer, a transparent layer, or no layer at all. In shutter mode, application of an AC electric field causes the capsule 20 to appear opaque. The transparency of a shutter mode display formed by the apparatus depicted in FIGS. 3A-3D may be controlled by the number of capsules addressed using DC fields and AC fields. For example, a display in which every other capsule 20 is addressed using an AC field would appear fifty percent transmissive.

FIGS. 3C and 3D depict an embodiment of the electrode structure described above in which electrodes 30, 40 are on “top” of the capsule 20, that is, the electrodes 30, 40 are between the viewpoint 10 and the capsule 20. In these embodiments, both electrodes 30, 40 should be transparent. Transparent polymers can be fabricated using conductive polymers, such as polyaniline, polythiophenes, or indium tin oxide. These materials may be made soluble so that electrodes can be fabricated using coating techniques such as spin coating, spray coating, meniscus coating, printing techniques, forward and reverse roll coating and the like. In these embodiments, light passes through the electrodes 30, 40 and is either absorbed by the particles 50, reflected by retroreflecting layer 60 (when provided), or transmitted throughout the capsule 20 (when retroreflecting layer 60 is not provided).

The addressing structure depicted in FIGS. 3A-3D may be used with electrophoretic display media and encapsulated electrophoretic display media. FIGS. 3A-3D depict embodiments in which electrode 30, 40 are statically attached to the display media. In certain embodiments, the particles 50 exhibit bistability, that is, they are substantially motionless in the absence of a electric field. In these embodiments, the electrodes 30, 40 may be provided as part of a “stylus” or other device which is scanned over the material to address each capsule or cluster of capsules. This mode of addressing particle-based displays will be described in more detail below in connection with FIG. 16.

Referring now to FIGS. 4A and 4B, a capsule 20 of a electronically addressable media is illustrated in which the technique illustrated above is used with multiple rear-addressing electrodes. The capsule 20 contains at least one particle 50 dispersed in a clear suspending fluid 25. The capsule 20 is addressed by multiple smaller electrodes 30 and multiple larger electrodes 40. In these embodiments, the smaller electrodes 30 should be selected to collectively be at most one-half the size of the larger electrodes 40. In further embodiments, the smaller electrodes 30 are collectively one-fourth the size of the larger electrodes 40. In further embodiments the smaller electrodes 30 are collectively one-eighth the size of the larger electrodes 40. In preferred embodiments, the smaller electrodes 30 are collectively one-sixteenth the size of the larger electrodes. Each electrode 30 may be provided as separate electrodes that are controlled in parallel to control the display. For example, each separate electrode may be substantially simultaneously set to the same voltage as all other electrodes of that size. Alternatively, the electrodes 30, 40 may be interdigitated to provide the embodiment shown in FIGS. 4A and 4B.

Operation of the rear-addressing electrode structure depicted in FIGS. 4A and 4B is similar to that described above. For example, the capsule 20 may contain positively charged, black particles 50 dispersed in a substantially clear suspending fluid 25. The smaller electrodes 30 are colored black and the larger electrodes 40 are colored white or are highly reflective. Referring to FIG. 4A, the smaller electrodes 30 are placed at a negative potential relative to the larger electrodes 40, causing particles 50 migrate within the capsule to the smaller electrodes 30 and the capsule 20 appears to the viewpoint 10 as a mix of the larger, white electrodes 40 and the smaller, black electrodes 30, creating an effect which is largely white. Referring to FIG. 4B, when the smaller electrodes 30 are placed at a positive potential relative to the larger electrodes 40, particles 50 migrate to the larger electrodes 40 causing the capsule 20 to display a mix of the larger, white electrodes 40 occluded by the black particles 50 and the smaller, black electrodes 30, creating an effect which is largely black. The techniques described above with respect to the embodiments depicted in FIGS. 1A and 1B for producing two-color displays work with equal effectiveness in connection with these embodiments.

FIGS. 5A and 5B depict an embodiment of a rear-addressing electrode structure that creates a reflective color display in a manner similar to halftoning or pointillism. The capsule 20 contains white particles 55 dispersed in a clear suspending fluid 25. Electrodes 42, 44, 46, 48 are colored cyan, magenta, yellow, and white respectively. Referring to FIG. 5A, when the colored electrodes 42, 44, 46 are placed at a positive potential relative to the white electrode 48, negatively-charged particles 55 migrate to these three electrodes, causing the capsule 20 to present to the viewpoint 10 a mix of the white particles 55 and the white electrode 48, creating an effect which is largely white. Referring to FIG. 5B, when electrodes 42, 44, 46 are placed at a negative potential relative to electrode 48, particles 55 migrate to the white electrode 48, and the eye 10 sees a mix of the white particles 55, the cyan electrode 42, the magenta electrode 44, and the yellow electrode 46, creating an effect which is largely black or gray. By addressing the electrodes, any color can be produced that is possible with a subtractive color process. For example, to cause the capsule 20 to display an orange color to the viewpoint 10, the yellow electrode 46 and the magenta electrode 42 are set to a voltage potential that is more positive than the voltage potential applied by the cyan electrode 42 and the white electrode 48. Further, the relative intensities of these colors can be controlled by the actual voltage potentials applied to the electrodes.

In another embodiment, depicted in FIG. 6, a color display is provided by a capsule 20 of size d containing multiple species of particles in a clear, dispersing fluid 25. Each species of particles has different optical properties and possess different electrophoretic mobilities (μ) from the other species. In the embodiment depicted in FIG. 6, the capsule 20 contains red particles 52, blue particles 54, and green particles 56, and |μ_(R)|

|μ_(B)|

|μ_(G)| That is, the magnitude of the electrophoretic mobility of the red particles 52, on average, exceeds the electrophoretic mobility of the blue particles 54, on average, and the electrophoretic mobility of the blue particles 54, on average, exceeds the average electrophoretic mobility of the green particles 56. As an example, there may be a species of red particle with a zeta potential of 100 millivolts (mV), a blue particle with a zeta potential of 60 mV, and a green particle with a zeta potential of 20 mV. The capsule 20 is placed between two electrodes 32, 42 that apply an electric field to the capsule.

FIGS. 7A-7B depict the steps to be taken to address the display shown in FIG. 6 to display a red color to a viewpoint 10. Referring to FIG. 7A, all the particles 52, 54, 56 are attracted to one side of the capsule 20 by applying an electric field in one direction. The electric field should be applied to the capsule 20 long enough to attract even the more slowly moving green particles 56 to the electrode 34. Referring to FIG. 7B, the electric field is reversed just long enough to allow the red particles 52 to migrate towards the electrode 32. The blue particles 54 and green particles 56 will also move in the reversed electric field, but they will not move as fast as the red particles 52 and thus will be obscured by the red particles 52. The amount of time for which the applied electric field must be reversed can be determined from the relative electrophoretic mobilities of the particles, the strength of the applied electric field, and the size of the capsule.

FIGS. 8A-8D depict addressing the display element to a blue state. As shown in FIG. 8A, the particles 52, 54, 56 are initially randomly dispersed in the capsule 20. All the particles 52, 54, 56 are attracted to one side of the capsule 20 by applying an electric field in one direction (shown in FIG. 8B). Referring to FIG. 8C, the electric field is reversed just long enough to allow the red particles 52 and blue particles 54 to migrate towards the electrode 32. The amount of time for which the applied electric field must be reversed can be determined from the relative electrophoretic mobilities of the particles, the strength of the applied electric field, and the size of the capsule. Referring to FIG. 8D, the electric field is then reversed a second time and the red particles 52, moving faster than the blue particles 54, leave the blue particles 54 exposed to the viewpoint 10. The amount of time for which the applied electric field must be reversed can be determined from the relative electrophoretic mobilities of the particles, the strength of the applied electric field, and the size of the capsule.

FIGS. 9A-9C depict the steps to be taken to present a green display to the viewpoint 10. As shown in FIG. 9A, the particles 52, 54, 56 are initially distributed randomly in the capsule 20. All the particles 52, 54, 56 are attracted to the side of the capsule 20 proximal the viewpoint 10 by applying an electric field in one direction. The electric field should be applied to the capsule 20 long enough to attract even the more slowly moving green particles 56 to the electrode 32. As shown in FIG. 9C, the electric field is reversed just long enough to allow the red particles 52 and the blue particles 54 to migrate towards the electrode 54, leaving the slowly-moving green particles 56 displayed to the viewpoint. The amount of time for which the applied electric field must be reversed can be determined from the relative electrophoretic mobilities of the particles, the strength of the applied electric field, and the size of the capsule.

In other embodiments, the capsule contains multiple species of particles and a dyed dispersing fluid that acts as one of the colors. In still other embodiments, more than three species of particles may be provided having additional colors. Although FIGS. 6-9C depict two electrodes associated with a single capsule, the electrodes may address multiple capsules or less than a full capsule

In FIG. 10, the rear substrate 100 for a seven segment display is shown that improves on normal rear electrode structures by providing a means for arbitrarily connecting to any electrode section on the rear of the display without the need for conductive trace lines on the surface of the patterned substrate or a patterned counter electrode on the front of the display. Small conductive vias through the substrate allow connections to the rear electrode structure. On the back of the substrate, these vias are connected to a network of conductors. This conductors can be run so as to provide a simple connection to the entire display. For example, segment 112 is connected by via 114 through the substrate 116 to conductor 118. A network of conductors may run multiple connections (not shown) to edge connector 122. This connector can be built into the structure of the conductor such as edge connector 122. Each segment of the rear electrode can be individually addressed easily through edge connector 122. A continuous top electrode can be used with the substrate 116.

The rear electrode structure depicted in FIG. 10 is useful for any display media, but is particularly advantageous for particle-based displays because such displays do not have a defined appearance when not addressed. The rear electrode should be completely covered in an electrically conducting material with room only to provide necessary insulation of the various electrodes. This is so that the connections on the rear of the display can be routed with out concern for affecting the appearance of the display. Having a mostly continuous rear electrode pattern assures that the display material is shielded from the rear electrode wire routing.

In FIG. 11, a 3×3 matrix is shown. Here, matrix segment 124 on a first side of substrate 116 is connected by via 114 to conductor 118 on a second side of substrate 116. The conductors 18 run to an edge and terminate in a edge connector 122. Although the display element of FIG. 11 shows square segments 124, the segments may be shaped or sized to form a predefined display pattern.

In FIG. 12, a printed circuit board 138 is used as the rear electrode structure. The front of the printed circuit board 138 has copper pads 132 etched in the desired shape. There are plated vias 114 connecting these electrode pads to an etched wire structure 136 on the rear of the printed circuit board 138. The wires 136 can be run to one side or the rear of the printed circuit board 138 and a connection can be made using a standard connector such as a surface mount connector or using a flex connector and anisotropic glue (not shown). Vias may be filled with a conductive substance, such as solder or conductive epoxy, or an insulating substance, such as epoxy.

Alternatively, a flex circuit such a copper-clad polyimide may be used for the rear electrode structure of FIG. 10. Printed circuit board 138 may be made of polyimide, which acts both as the flex connector and as the substrate for the electrode structure. Rather than copper pads 132, electrodes (not shown) may be etched into the copper covering the polyimide printed circuit board 138. The plated through vias 114 connect the electrodes etched onto the substrate the rear of the printed circuit board 138, which may have an etched conductor network thereon (the etched conductor network is similar to the etched wire structure 136).

In FIG. 12, a thin dielectric sheet 150, such as polyester, polyimide, or glass can be used to make a rear electrode structure. Holes 152 are punched, drilled, abraded, or melted through the sheet where conductive paths are desired. The front electrode 154 is made of conductive ink printed using any technique described above. The holes should be sized and the ink should be selected to have a viscosity so that the ink fills the holes. When the back structure 156 is printed, again using conductive ink, the holes are again filled. By this method, the connection between the front and back of the substrate is made automatically.

In FIG. 14, the rear electrode structure can be made entirely of printed layers. A conductive layer 166 can be printed onto the back of a display comprised of a clear, front electrode 168 and a printable display material 170. A clear electrode may be fabricated from indium tin oxide or conductive polymers such as polyanilines and polythiophenes. A dielectric coating 176 can be printed leaving areas for vias. Then, the back layer of conductive ink 178 can be printed. If necessary, an additional layer of conductive ink can be used before the final ink structure is printed to fill in the holes.

This technique for printing displays can be used to build the rear electrode structure on a display or to construct two separate layers that are laminated together to form the display. For example an electronically active ink may be printed on an indium tin oxide electrode. Separately, a rear electrode structure as described above can be printed on a suitable substrate, such as plastic, polymer films, or glass. The electrode structure and the display element can be laminated to form a display.

Referring now to FIG. 15, a threshold may be introduced into an electrophoretic display cell by the introduction of a third electrode. One side of the cell is a continuous, transparent electrode 200 (anode). On the other side of the cell, the transparent electrode is patterned into a set of isolated column electrode strips 210. An insulator 212 covers the column electrodes 210, and an electrode layer on top of the insulator is divided into a set of isolated row electrode strips 230, which are oriented orthogonal to the column electrodes 210. The row electrodes 230 are patterned into a dense array of holes, or a grid, beneath which the exposed insulator 212 has been removed, forming a multiplicity of physical and potential wells.

A positively charged particle 50 is loaded into the potential wells by applying a positive potential (e.g. 30V) to all the column electrodes 210 while keeping the row electrodes 230 at a less positive potential (e.g. 15V) and the anode 200 at zero volts. The particle 50 may be a conformable capsule that situates itself into the physical wells of the control grid. The control grid itself may have a rectangular cross-section, or the grid structure may be triangular in profile. It can also be a different shape which encourages the microcapsules to situate in the grid, for example, hemispherical.

The anode 200 is then reset to a positive potential (e.g. 50V). The particle will remain in the potential wells due to the potential difference in the potential wells: this is called the Hold condition. To address a display element the potential on the column electrode associated with that element is reduced, e.g. by a factor of two, and the potential on the row electrode associated with that element is made equal to or greater than the potential on the column electrode. The particles in this element will then be transported by the electric field due to the positive voltage on the anode 200. The potential difference between row and column electrodes for the remaining display elements is now less than half of that in the normal Hold condition. The geometry of the potential well structure and voltage levels are chosen such that this also constitutes a Hold condition, i.e., no particles will leave these other display elements and hence there will be no half-select problems. This addressing method can select and write any desired element in a matrix without affecting the pigment in any other display element. A control electrode device can be operated such that the anode electrode side of the cell is viewed.

The control grid may be manufactured through any of the processes known in the art, or by several novel processes described herein. That is, according to traditional practices, the control grid may be constructed with one or more steps of photolithography and subsequent etching, or the control grid may be fabricated with a mask and a “sandblasting” technique.

In another embodiment, the control grid is fabricated by an embossing technique on a plastic substrate. The grid electrodes may be deposited by vacuum deposition or sputtering, either before or after the embossing step. In another embodiment, the electrodes are printed onto the grid structure after it is formed, the electrodes consisting of some kind of printable conductive material which need not be clear (e.g. a metal or carbon-doped polymer, an intrinsically conducting polymer, etc.).

In a preferred embodiment, the control grid is fabricated with a series of printing steps. The grid structure is built up in a series of one or more printed layers after the cathode has been deposited, and the grid electrode is printed onto the grid structure. There may be additional insulator on top of the grid electrode, and there may be multiple grid electrodes separated by insulator in the grid structure. The grid electrode may not occupy the entire width of the grid structure, and may only occupy a central region of the structure, in order to stay within reproducible tolerances. In another embodiment, the control grid is fabricated by photoetching away a glass, such as a photostructural glass.

In an encapsulated electrophoretic image display, an electrophoretic suspension, such as the ones described previously, is placed inside discrete compartments that are dispersed in a polymer matrix. This resulting material is highly susceptible to an electric field across the thickness of the film. Such a field is normally applied using electrodes attached to either side of the material. However, as described above in connection with FIGS. 3A-3D, some display media may be addressed by writing electrostatic charge onto one side of the display material. The other side normally has a clear or opaque electrode. For example, a sheet of encapsulated electrophoretic display media can be addressed with a head providing DC voltages.

In another implementation, the encapsulated electrophoretic suspension can be printed onto an area of a conductive material such as a printed silver or graphite ink, aluminized Mylar, or any other conductive surface. This surface which constitutes one electrode of the display can be set at ground or high voltage. An electrostatic head consisting of many electrodes can be passed over the capsules to addressing them. Alternatively, a stylus can be used to address the encapsulated electrophoretic suspension.

In another implementation, an electrostatic write head is passed over the surface of the material. This allows very high resolution addressing. Since encapsulated electrophoretic material can be placed on plastic, it is flexible. This allows the material to be passed through normal paper handling equipment. Such a system works much like a photocopier, but with no consumables. The sheet of display material passes through the machine and an electrostatic or electrophotographic head addresses the sheet of material.

In another implementation, electrical charge is built up on the surface of the encapsulated display material or on a dielectric sheet through frictional or triboelectric charging. The charge can built up using an electrode that is later removed. In another implementation, charge is built up on the surface of the encapsulated display by using a sheet of piezoelectric material.

FIG. 16 shows an electrostatically written display. Stylus 300 is connected to a positive or negative voltage. The head of the stylus 300 can be insulated to protect the user. Dielectric layer 302 can be, for example, a dielectric coating or a film of polymer. In other embodiments, dielectric layer 302 is not provided and the stylus 300 contacts the encapsulated electrophoretic display 304 directly. Substrate 306 is coated with a clear conductive coating such as ITO coated polyester. The conductive coating is connected to ground. The display 304 may be viewed from either side.

Microencapsulated displays offer a useful means of creating electronic displays, many of which can be coated or printed. There are many versions of microencapsulated displays, including microencapsulated electrophoretic displays. These displays can be made to be highly reflective, bistable, and low power.

To obtain high resolution displays, it is useful to use some external addressing means with the microencapsulated material. This invention describes useful combinations of addressing means with microencapsulated electrophoretic materials in order to obtain high resolution displays.

One method of addressing liquid crystal displays is the use of silicon-based thin film transistors to form an addressing backplane for the liquid crystal. For liquid crystal displays, these thin film transistors are typically deposited on glass, and are typically made from amorphous silicon or polysilicon. Other electronic circuits (such as drive electronics or logic) are sometimes integrated into the periphery of the display. An emerging field is the deposition of amorphous or polysilicon devices onto flexible substrates such as metal foils or plastic films.

The addressing electronic backplane could incorporate diodes as the nonlinear element, rather than transistors. Diode-based active matrix arrays have been demonstrated as being compatible with liquid crystal displays to form high resolution devices.

There are also examples of crystalline silicon transistors being used on glass substrates. Crystalline silicon possesses very high mobilities, and thus can be used to make high performance devices. Presently, the most straightforward way of constructing crystalline silicon devices is on a silicon wafer. For use in many types of liquid crystal displays, the crystalline silicon circuit is constructed on a silicon wafer, and then transferred to a glass substrate by a “liftoff” process. Alternatively, the silicon transistors can be formed on a silicon wafer, removed via a liftoff process, and then deposited on a flexible substrate such as plastic, metal foil, or paper. As another implementation, the silicon could be formed on a different substrate that is able to tolerate high temperatures (such as glass or metal foils), lifted off, and transferred to a flexible substrate. As yet another implementation, the silicon transistors are formed on a silicon wafer, which is then used in whole or in part as one of the substrates for the display.

The use of silicon-based circuits with liquid crystals is the basis of a large industry. Nevertheless, these display possess serious drawbacks. Liquid crystal displays are inefficient with light, so that most liquid crystal displays require some sort of backlighting. Reflective liquid crystal displays can be constructed, but are typically very dim, due to the presence of polarizers. Most liquid crystal devices require precise spacing of the cell gap, so that they are not very compatible with flexible substrates. Most liquid crystal displays require a “rubbing” process to align the liquid crystals, which is both difficult to control and has the potential for damaging the TFT array.

The combination of these thin film transistors with microencapsulated electrophoretic displays should be even more advantageous than with liquid crystal displays. Thin film transistor arrays similar to those used with liquid crystals could also be used with the microencapsulated display medium. As noted above, liquid crystal arrays typically requires a “rubbing” process to align the liquid crystals, which can cause either mechanical or static electrical damage to the transistor array. No such rubbing is needed for microencapsulated displays, improving yields and simplifying the construction process.

Microencapsulated electrophoretic displays can be highly reflective. This provides an advantage in high-resolution displays, as a backlight is not required for good visibility. Also, a high-resolution display can be built on opaque substrates, which opens up a range of new materials for the deposition of thin film transistor arrays.

Moreover, the encapsulated electrophoretic display is highly compatible with flexible substrates. This enables high-resolution TFT displays in which the transistors are deposited on flexible substrates like flexible glass, plastics, or metal foils. The flexible substrate used with any type of thin film transistor or other nonlinear element need not be a single sheet of glass, plastic, metal foil, though. Instead, it could be constructed of paper. Alternatively, it could be constructed of a woven material. Alternatively, it could be a composite or layered combination of these materials.

As in liquid crystal displays, external logic or drive circuitry can be built on the same substrate as the thin film transistor switches.

In another implementation, the addressing electronic backplane could incorporate diodes as the nonlinear element, rather than transistors.

In another implementation, it is possible to form transistors on a silicon wafer, dice the transistors, and place them in a large area array to form a large, TFT-addressed display medium. One example of this concept is to form mechanical impressions in the receiving substrate, and then cover the substrate with a slurry or other form of the transistors. With agitation, the transistors will fall into the impressions, where they can be bonded and incorporated into the device circuitry. The receiving substrate could be glass, plastic, or other nonconductive material. In this way, the economy of creating transistors using standard processing methods can be used to create large-area displays without the need for large area silicon processing equipment.

The following Example is now given, though by way of illustration only, to show details of particularly preferred reagents, conditions and techniques used in the electrophoretic media and displays of the present invention.

EXAMPLE

An internal phase was prepared comprising 10 percent by volume white particles and 1 percent by volume black particles (carbon black) by volume in a hydrocarbon suspending fluid; the internal phase had a viscosity of 4.75 mPa sec. The white particles comprised titania and had an average size of approximately 0.6 μm and a saturation particle thickness estimated at 1.5 to 2.5 μm. The internal phase was encapsulated in gelatin/acacia microcapsules substantially as described in Paragraphs [0069] to [0074] of U.S. Patent Publication No. 2002/0180687. The resultant microcapsules were separated into three batches differing in wet capsule diameter size distributions. Each batch was mixed into a slurry with a polymeric binder, coated to form an electrophoretic film, and laminated to a back electrode to form a switchable display pixel, substantially as described in Paragraphs [0075] and [0076] of the aforementioned 2002/0180687. During the coating process, suitable equipment settings such as speed, pressure and die height were used to achieve a range of wet film coat weights, which then dried into capsules of differing IP heights in part due to the effects of binder evaporation and surface tension. In one batch, the dry capsules were roughly spherical; in the second batch the dry capsules had substantially the form of oblate spheroids; and in the third batch the dry capsules had substantially the form of prolate spheroids, with heights greater than their diameters. In each case the spheroids ranged from circular in XY projection to hexagonal, varying with the packing density in the film of electrophoretic medium.

The three resultant electrophoretic media differed in estimated IP heights and typical pixel optical properties when switched with a 350 ms 15 V pulse, as shown in the Table below.

TABLE Estimated IP Maximum Height in White Dark Contrast Visual Microcavity (μm) State L* State L* Ratio Artifacts L* 18 62 21 9.4:1 1.5 44 59 27 5.3:1 3.7 55 60 24 6.9:1 5.5

Assuming a saturation particle thickness for the titania of 2 μm, the optimum IP height according to the present invention should be 20 μm; the IP height calculated for the black pigment is substantially less, so that it is the optimum IP height for the titania which is important for this medium. It will be seen from the data in the Table above that the capsules having an estimated IP height of 18 μm, close to the calculated 20 μm, had substantially better optical properties, including an improved contrast ratio, as compared with the other two media having substantially greater IP heights.

It will be apparent to those skilled in the art that numerous changes and modifications can be made in the specific embodiments of the invention described above without departing from the scope of the invention. Accordingly, the whole of the foregoing description is to be interpreted in an illustrative and not in a limitative sense, the invention being defined solely by the appended claims. 

1. An electrophoretic medium having walls defining at least one microcavity containing an internal phase, this internal phase comprising a plurality of at least one type of electrophoretic particle suspended in a suspending fluid and capable of moving therethrough upon application of an electric field to the electrophoretic medium, the average height of the at least one microcavity differing by not more than about 5 μm from the saturated particle thickness of the electrophoretic particle divided by the volume fraction of the electrophoretic particles in the internal phase.
 2. An electrophoretic medium according to claim 1 wherein the saturated particle thickness is between about 1 and about 5 μm.
 3. An electrophoretic medium according to claim 2 wherein the saturated particle thickness is between about 1.5 and about 2.5 μm.
 4. An electrophoretic medium according to claim 1 such that, if application of a specific electric field to the medium for time T suffices to switch the medium between its extreme optical states, variations in the time of application of this specific electric field within the range of 0.9 to 1.1 T will not change the optical properties of either extreme state of the electrophoretic medium by more than 2 units of L*.
 5. An electrophoretic medium according to claim 4 such that variations in the time of application of this specific electric field within the range of 0.8 to 1.2 T will not change the optical properties of either extreme state of the electrophoretic medium by more than 2 units of L*.
 6. A electrophoretic medium according to claim 1 comprising a single type of electrophoretic particle in a colored suspending fluid.
 7. A electrophoretic medium according to claim 1 comprising a first type of electrophoretic particle having a first optical characteristic and a first electrophoretic mobility and a second type of electrophoretic particle having a second optical characteristic different from the first optical characteristic and a second electrophoretic mobility different from the first electrophoretic mobility.
 8. A electrophoretic medium according to claim 7 wherein the suspending fluid is uncolored.
 9. A electrophoretic medium according to claim 1 wherein the electrophoretic particles and the suspending fluid are retained within a plurality of cavities formed within a carrier medium.
 10. A electrophoretic medium according to claim 1 wherein the electrophoretic particles and the suspending fluid are held within a plurality of capsules.
 11. An electrophoretic medium according to claim 1 wherein the electrophoretic particles comprise titania.
 12. An electrophoretic medium according to claim 11 wherein the electrophoretic particles further comprise dark colored particles formed from carbon black or copper chromite, the dark colored particles formed from carbon black or copper chromite and having an electrophoretic mobility different from the electrophoretic mobility of the titania particles.
 13. An electrophoretic medium according to claim 1 wherein the volume fraction of electrophoretic particles in the internal phase is from about 3 to about 40 per cent.
 14. An electrophoretic medium according to claim 13 wherein the volume fraction of electrophoretic particles in the internal phase is from about 6 to about 18 per cent.
 15. An electrophoretic medium according to claim 1 having an internal phase height between about 10 and about 30 μm and a volume fraction of electrophoretic particles of between about 3 and about 15 per cent.
 16. An electrophoretic medium according to claim 15 having an internal phase height between about 12 and about 25 μm and a volume fraction of electrophoretic particles of between about 5 and about 12 per cent.
 17. An electrophoretic medium according to claim 1 wherein the viscosity of the internal phase is less than about 5 mPa sec.
 18. An electrophoretic medium according to claim 17 wherein the viscosity of the internal phase is greater than about 1 mPa sec.
 19. An electrophoretic display comprising an electrophoretic medium according to claim 1 and at least one electrode disposed adjacent the electrophoretic medium and arranged to apply an electric field thereto.
 20. An electrophoretic display according to claim 19 having a first optical state in which the display displays an optical characteristic of the one type of electrophoretic particle and a second optical state in which the electrophoretic medium is light-transmissive.
 21. An electrophoretic display according to claim 20 wherein, in the light-transmissive optical state, the electrophoretic particles are confined in a minor proportion of the cross-sectional area of each microcavity.
 22. An electrophoretic display according to claim 20 comprising a backplane comprising a plurality of pixel electrodes, and a color filter or reflector, the color filter or reflector being disposed between the backplane and the electrophoretic medium. 